Image analysis is concerned with extracting data from images using electronic techniques. Some of these techniques have been described in detail in the R. A. Morton U.S. Pat. No. 3,805,028. That patent is concerned primarily with extracting information relating to various measurements of geometric size of features or objects in the field of view.
In this disclosure, the objects to be measured, generically termed features, which may correspond to particles, lakes, cells, islands, or other objects depending on the image being analyzed, are processed to determine the value of parameters relating to density, transmittance, absorbance, and reflectance. These measurements are generally unrelated to size, but are related to the intensity, or brightness, or darkness of the feature. These intensity related measurements are derived from the amplitude of the video signal generated by the television camera or scanner which scans the image traversing the features of interest.
In order to get a complete and detailed characterization of a specific object not only are its size, shape and other geometric measures of importance, but also a characterization of its intensity or gray level is important, for often it contains significant information to be further used, for example, to determine measurements relating to volume of particles and cells composed of semi-transparent material.
The magnitude of these amplitude related measurements is influenced by how the object is imaged, whether using, for example, transmitted or reflected light. Also, the spectral characteristics of the illumination, as well as the spectral characteristics of the pickup tube of the television scanner which scans the image of the object, and the angles of illumination of the object, as well as the angles at which light is rceived from the object, all have an effect on the measurement obtained and must therefore be taken into account in performing the measurement. For example, a partially opaque object when viewed in transmitted light has an integrated density which is related to its specific optical density value through Beer's law which states that the density of each point of an object is proportional to its specific optical density times its thickness. Similarly, the reflectance of an object when viewed in incident light can under appropriate conditions be related back to the complex refractive index of that object.
In order to perform measurements based on the magnitudes of video amplitudes it is first necessary to scan the object to obtain video signals representing the object and its field. This important proces produces a video signal having a number of limiting characteristics which, if not corrected for, can introduce significant errors into the final result. The first of these characteristics is the possible nonlinear relationship between the amplitude of the video signal and the intensity of the observed image. For example, although a Plumbicon .RTM. pickup tube produces a linear response of output signal with light intensity, the vidicon scanner tube does not. The vidicon scanner, on the other hand, exhibit a greater sensitivity in the dark end of the video range and can, therefore, provide a wider dynamic range than the Plumbicon .RTM. tube.
In deriving meaningful measures of density, transmittance or reflectance from the video signal, the relationship between the video amplitude response of the scanner and image intensity must be taken into account directly or indirectly. This invention provides a way to accommodate for the variation between different scanner tubes and simultaneously accommodate for the fact that density measurements, for example, vary in a different way with the video amplitude than do transmittance measurements, by using a memory to store the overall functional relationship between the video signals and required measurement, whether it be, for example, density or transmittance. This overall functional relationship takes into account the linearity characteristics of a pickup tube as well as the sensitivity characteristics of the optical system and illumination. Because different pickup tubes may be used and different measurements (e.g., density or absorbance or transmittance) may be measured using the same system, it is desirable to be able to change this functional relationship to suit the desired measurement and the available scanning tube.